What makes a novel gripping? Many readers will know the experience of setting out on the latest Ian McEwan novel and feeling caught – compelled. So it is with Sweet Tooth: the sense of narrative purpose exerts its pull from the first. Our usual word for this is "plot", to whose pleasures we succumb when we recognise the signs of a concealed – but slowly revealed – design in a novel. With what is sometimes called a postmodernist novel, our sense of purpose is rather different. Here design has to do less with the intriguing behaviour of the characters (what are these people really up to?) than with the genesis of the narration. What will give the story its shape?

The opening paragraph signals the trajectory of the narrative, as is conventional in a first-person account. McEwan's narrator, Serena Frome, does not disguise the fact that she already knows how her story will end: "almost forty years ago I was sent on a secret mission for the British security service. I didn't return safely". Yet there is something unusual here too. In what we will soon recognise as her cool manner, she jokes about her fate. To be telling her story she must surely have returned safely. She sounds as if she is referring to a deadly incursion behind the iron curtain; we will soon find, however, that she is sent to beguile a novelist living in Brighton.

Serena's peculiarity (certainly in the eyes of her MI5 bosses) is that she is addicted to fiction. She believes herself to be "a good reader". Yet her tastes are particular. Her confession that she craves a "naive realism" is a clue to her eventual fate. "I suppose I would not have been satisfied until I had in my hands a novel about a girl in a Camden bedsit who occupied a lowly position in MI5 and was without a man." This is the novel that you are reading at that moment. A postmodernist trick? As someone who despises postmodernism, Serena is doomed to be caught in its toils. "I said I didn't like tricks, I liked life as I knew it re-created on the page." Her antagonism feels like a joke (an Ian McEwan novel would surely irk her), but it is really a warning.

And there are the tricks for us all to see. As Henry Perowne in Saturday seemed to have been given Ian McEwan's Fitzrovia home, Tom Haley is given some of his biography. He shows his early stories to the literary editor Ian Hamilton, who appears as his recognisable self in a crucial scene. Haley's eventual publisher is McEwan's own, Tom Maschler, the Svengali of British literary fiction. (Without our all-knowing retrospect, Haley cannot quite get his name right.) We get an animated sketch of him – complete with delighted literary name‑dropping – when Serena and Tom visit his Bloomsbury office. At his first public reading he shares a stage with (and is duly upstaged by) McEwan's friend, a young Martin Amis.

McEwan is demonstrating what Serena will find out: the novelist lives to accumulate material. As Philip Roth's alter ego Nathan Zuckerman describes it, "Feeding that great opportunistic maw, a novelist's mind". When Haley's stories are inserted, in Serena's summaries, into the novel, some bear a passing similarity to some of McEwan's own early stories. But also these stories will turn out each to be some parable of Serena's fate. As we read, their eventual significance cannot yet be clear, but (unlike Serena, the would-be empiricist) we know – we trust – that they are not just virtuoso flourishes.

The story of Sweet Tooth is about spies, so we are schooled in the inference that all information it provides – especially circumstantial information – is likely to be significant. Tom Haley, the aspiring novelist whom Serena is to recruit, gets to see the analogy between the two professions. "I too could be a spy," he says, thinking of how he comes to use relationships as the material for his fiction. When he and Serena become lovers, she harbours an inevitable suspicion. "I couldn't banish the thought that he was quietly recording our lovemaking for future use, that he was making mental notes." Haley later repeats a story told him by Ian Hamilton (and therefore, we guess, told the young McEwan by the same source) of a writer whose wife left him because he "put intimate details of his marriage into a novel".

Serena should know – she does half-know – that she puts herself at risk in consorting with a novelist. Just as much as, say, Great Expectations or The Good Soldier, Sweet Tooth grips the reader with the conviction that the narrator is the victim of elaborate deception. Those who have no time for narrative trickery will find themselves caught out by it. It gives little away to say that at the end of the novel its protagonist must choose whether to relish or deplore the fact that she has been manipulated by a writer of fiction. The reader who has been gripped by Sweet Tooth has, by definition, already decided that being expertly manipulated by the novelist is a reader's privilege.